The first objective of this project s to produce a public use file of individual and household records drawn from the Puerto Rican censuses of 1910 and 1920 (PURIPUS 1910 and 1920). The public use file will be exactly analogous and strictly comparable to the US public use samples created as part of the IPUMS project (Ruggles and Sobeck, 1995). The public use file will also include a sample of individuals and household records of persons of Puerto Rican origin residing in the state of New York and enumerated in the corresponding US censuses. A second objective of the proposed project is to pursue three different interpretative tasks. The first consists of utilizing the social, economic and demographic information from the public use files to describe in detail the origins of one of the most sudden and rapid demographic changes that took place among developing countries in general and Latin American in particular and, to evaluate the factors precipitated it. More concretely, we will: (a) describe levels and patterns of nuptiality, fertility and marital fertility between 1900 and 1920 and (b) assess levels and patterns of infant and child mortality, during the same period. The second task is to document and understand the nature and determinants of changes in household organization that took place during the period 1910-1920. As deep transformations were taking place in the economy, modernization theory expects that household structure would experience important transformations. According to local historical accounts, these changes did indeed take place but we do not know how localized they were, or if indeed they engulfed the entire country, whether they occurred as a result of changes in individual values and preferences or due to shifts in demographic constraints. Through the use of microsimulation and the census information on household structure we will attempt to document the magnitude of changes, the nature of regional differentials and the factors accounting for them. Finally, we will systematically compare levels and patterns of fertility, mortality and household formation in the population living in the mainland and those living in the island. These comparisons will shed light on the nature and importance of assimilation processes.